Compressed natural gas (CNG) vehicles require specialized refueling delivery systems. One such system utilizes high pressure storage vessels or bottles which are delivered full to a dispensing or filling station on shuttle trucks. A hose from the storage vessel is connected to the tank on the vehicle to be refueled, allowing CNG to flow from the storage vessel to the tank. A high pressure compressor applies pressure to the vessels as the CNG fuel flows to the vehicles. The compressor is required to compensate for pressure drops in the vessels resulting from dispensing the CNG.
This system has several disadvantages. The compressor is very expensive and usually represents a significant portion of the cost of the filling station. The compressor is also noisy, it requires regular maintenance and the system is inefficient. The compressor can only unload approximately 50% of the CNG contained within the high pressure vessels due to its fixed compression ratio.
Another CNG refueling system consists of equipping the filling stations with hydraulic power units (HPU). The shuttle trucks carrying CNG in pressure vessels are connected to a hydraulic system. The pressure vessels have an internal piston which is pressurized by hydraulic fluid. The piston also separates the hydraulic fluid from the CNG. HPU pumps are used to maintain pressure in the vessels as CNG is dispensed.
This type of system also has several disadvantages. The pressure vessels require large volumes of hydraulic fluid from a large hydraulic fluid reservoir. The large quantity of hydraulic fluid requires significant power handling capabilities, it must be preheated in colder climates, and it poses a more serious pollution hazard. Once the vessels are filled with hydraulic fluid, a significant amount of time is required to drain the vessels.